1. Field of the Invention
The present invention relates to a method and a device for measuring optical properties of an optically variable marking applied on an object, light coming from the optically variable marking having different spectral compositions when viewed from different angles.
2. Brief Description of the Related Art
In the field of optical inspection of labels, markings, and visible tags, usually a camera system having a lens and an image sensor is used, coupled to a frame grabber and a computer system. The image sensor can be triggered to capture an image when an object carrying the label, marking, or visible code passes into the field of view of the camera system. While the object passes, the label of the object can be captured by the image sensor after a special triggering event, for example by a motion detection sensor so that a digital image of the label can be formed. The image sensor will thereby operate with fast integration times to generate a blur-free image, and usually has the necessary driver and signal electronics to convert the captured light by analog-to-digital conversion into a digital image. The digital image may be held and formatted by the frame grabber, and can be passed on for further processing to a computer system, by using specific image processing algorithms. For example, optical measurements of dimensions of the object can be performed, and optical character recognition or pattern matching algorithms can be used to detect and read certain information that is represented by the labels and markings.
Moreover, devices have been proposed to inspect labels and markings that are made from optical variable ink by a human observer, without the use of camera systems for automated inspection. These devices have proposed to use mirrors installed in a viewing device to reflect light that has been emitted from the optical variable marking, so that the human observer can look at the optical variable marking from a fixed spatial orientation. This allows the observer to see simultaneously the image of the optical variable marking in first and second colors without movement of the observer.
Despite all of the technologies in the field of optical inspection systems, in the field of measuring properties of optically variable markings, dedicated optical inspection solutions are still needed, to improve costs, performance and versatility of these systems.